The present invention relates generally to nerve electrodes, and more particularly to an improved circumneural electrode assembly of added strength and flexibility for implantation on and electrical stimulation of a selected nerve or nerve bundle of a patient.
Circumneural electrodes are generally designed to encompass a portion of a nerve longitudinally to permit electrical stimulation of the nerve. The stimulation may be intended to modulate electrical signals or impulses normally carried by the nerve. Alternatively or additionally, the nerve electrode may be used for sensing electrical signals carried by the nerve. The required installation of the electrode on a nerve for such purposes presents a considerable number of design problems. To provide mechanical stability of the electrode relative to the nerve, and in recognition that the nerve can move relative to the surrounding tissue, a structure which encompasses the nerve is desirable, and provides efficiency in minimizing or optimizing the distance between the stimulating electrode and the nerve body. It is axiomatic, however, that nerves are sensitive and are easily damaged or traumatized by abrasion or stresses caused by subjection to mechanical forces.
Such forces may be attributable to constriction of the nerve by the circumneural electrode, or to pulling or torque transmitted to the electrode (and thus, to the nerve) by a lead wire. Or the nerve may atrophy as a consequence of lack of nutritional fluid exchange owing to the close proximity of the electrode. Cuff electrodes were popular as nerve electrodes at one time, but lost much of their original appeal because they were found to be too stiff--their rigidity often causing nerve damage.
U.S. Pat. No. 4,573,481 ("the '481 patent") discloses an implantable helical electrode assembly in which the configuration is composed of one or more flexible ribbon electrodes each partially embedded in a portion of the peripheral surface of an open helical dielectric support matrix adapted to be threaded or wrapped around a selected nerve or nerve bundle during surgical implantation of the electrode assembly. The resiliency of the assembly allows it to expand in the event of swelling of the nerve. The electrode assembly is utilized to electrically trigger or measure an action potential or to block conduction in nerve tissue.
Such a helical electrode, with its expansion characteristic, allows "one size" to fit a multitude of variations in nerve diameter encountered in different patients at a given nerve stimulation site. It also allows fluid exchange between the helical coils and is mechanically compliant at its ends. However, some difficulty may be experienced in attempting to install the configuration on the patient's nerve, because it is necessary to unravel the helical configuration and then reform it about the nerve. An improvement over the '481 patent electrode design is disclosed in U.S. Pat. No. 4,920,979, in which a flexible electrode-supporting matrix has two oppositely directed helical portions which are centrally joined and have free outer ends. The helical portions extend circumferentially at least one turn and up to as much as about two turns. A thin, flexible conductive ribbon is secured to the inner surface to provide multiple electrodes on one or both portions, with a connecting electrical cable to couple the electrode array to an electronics package intended for stimulation and/or sensing, implanted elsewhere in the body.
In the design of the '979 patent, the central passage through the two oppositely directed helical portions accommodates a pair of pins which extend at an acute angle from the respective closed legs of a tweezer-like installation tool. When the pins are inserted through the central passage and the legs of the tweezers are opened, the helical portions are distorted and spread open so that the assembly can be slipped over the nerve with the two open-sided portions restrained in a direction generally perpendicular to the length of the nerve. When released by withdrawing the pins of the installation tool, the two end portions return to a helical shape to encircle the nerve with their electrode portions conductively contacting the nerve surface. This type of electrode assembly simplifies installation of the electrode and reduces trauma to the nerve during implantation.
Despite the availability of these and other circumneural helical or spiral electrodes, problems remain in the attachment of the lead wire to the electrode assembly. A particular problem lies in attempts to address conflicting design goals of maximizing mechanical strength for fatigue resistance while minimizing spring constant to allow compliance with the nerve and its movement. It is desirable to improve the strength, durability, flexibility and fatigue resistance of the electrode assembly itself, and as well, to improve the mechanical strength of the electrical connection between the lead conductor and the electrode assembly.
FIG. 1 illustrates a lead 10 of a type heretofore available for implantation in the human body for use in nerve stimulation, including a lead body 11 having lead connectors 12 at its proximal end, and a helical or spiral electrode assembly 13 at its distal end. The connectors 12 are designed to mate with female electrical connectors of an implanted generator (not shown) of electrical signals for electrical stimulation of the nerve and/or for sensing the electrical signals carried by the nerve. The lead body 11 typically comprises an MP35N (cobalt chromium alloy) electrical coil conductor with a biocompatible electrically insulative sheath.
As will be described in greater detail with reference to FIGS. 2 and 3 below, the electrode assembly 13 includes one or more single turn platinum spiral electrical stimulation ribbon conductors with a 90% platinum/10% iridium alloy wire reinforcing component in the weld between the conductor coil and the ribbon electrode. The ribbon conductor is molded in a silicone elastomer insulating material so that the conductor portion is bonded to the insulation but exposed at the underside of the spiral. An integral anchor tether 15 is employed to retain the implanted electrode in place without undue flexing, thereby substantially reducing the possibility of fatigue and fracture of the electrode or the weld connection to the conductor coil.
In this prior art design of FIG. 1, the platinum ribbon used for the stimulating electrode surface is heat treated or annealed, which makes the platinum material soft and ductile. These properties make the lead, and especially the ribbon electrode, vulnerable to damage during implantation as a result of excessive manipulation or improper handling. For example, during installation on the nerve, the electrode helix may be overly stretched by the surgeon to leave it in a deformed condition. Deformation of the platinum material for this or any other cause potentially affects its performance and long term reliability as a nerve-stimulating electrode.
For the sake of clarity, certain details of the prior design of the electrode assembly 13 are shown in FIG. 2. The assembly 13 includes an electrode helix consisting of a single turn of platinum ribbon 20 bonded (e.g., molded) to the inner surface of a silicone elastomer helix 21 which continues through one additional turn 22 and 23 at either end of the platinum ribbon 20 to form a three-turn helix in which each of the single end turns is simply an extension of the central elastomer portion to which the ribbon conductor 20 is bonded. The molding process may utilize injection molding or flow molding, or other known techniques, for example. The coil conductor 25 of lead 10 is welded at 27 to ribbon electrode 20 prior to the molding of the ribbon in the silicone elastomer to achieve the helical shape. A silicone sheath 28 covers the entire length of the lead body. A suture 24 runs the length of the helical configuration.
Details of the welded ribbon electrode subassembly 30 of the prior design are illustrated in FIG. 3. The structure is shown prior to being molded into a helical structure, and the silicone insulating material is not shown, for the sake of clarity. The MP35N quadfilar coil conductor 25 has a 0.25 mm outer diameter, and has an end welded at joint 27 to the 0.025 mm thick by 1.0 mm wide annealed platinum (99.95%) ribbon 20, and then formed into a radius bend 36. The ribbon is heat-treated in an alcohol flame except in the area of the weld. Heat-treated platinum is more ductile, but suffers from lower tensile strength and lower fatigue resistance. The ribbon portion is subsequently molded in silicone elastomer. The ribbon has flaps 31, 32 at its ends, and suture holes 34, 35 at the ends at or very near the point at which the flap commences.
Although the tether 15 (FIG. 1) reduces the magnitude of repetitive, albeit small force loads that exist on the lead connection to the electrode after implantation, there remains the possibility of mechanical fatigue at the weld joint 27. Also, the lead body (sheathed coil conductor) should exit from the helical structure above (about 1.25 mm, in the configuration described above) the molded ribbon conductor to avoid interference with the adjacent spiral structure as well as contact with the nerve and resulting abrasions. However, the radius bend 36 of the lead coil conductor 25 tends to exacerbate the problem, by creating a lever action at the weld joint. Also, the welding process itself heat treats the area in the immediate vicinity of the weld, thereby tending to reduce the strength of the weld. The transition zone between the stronger non-heat treated area and the weaker heat treated areas has the potential for fatigue.
It is a principal object of the present invention to provide an improved lead and electrode, or electrode assembly, for nerve stimulation. Consistent with that object, the electrode assembly is configured for relative ease of implantation and reliable retention on the nerve, while providing improved flexibility and mechanical strength and fatigue resistance of the lead/electrode connection relative to nerve electrodes of the prior art.
Another object of the invention is to provide an improved nerve lead and electrode assembly which is configured in a way that avoids abrasion or other damage to the nerve either from the electrode assembly or the lead conductor itself, while assuring good retention and flexibility characteristics.
Yet another object of the invention is to provide an improved lead and electrode assembly for nerve stimulation in which the electrode design minimizes tissue ingrowth at the welded coil region, between the ribbon and elastomer interface.